1. Technical Field
The present invention relates to the cleaning of equipment in which chemical mechanical planarization (“CMP”) is performed. In particular, the present invention relates to chemical compositions for cleaning CMP equipment wherein said compositions have improved cleaning performance and/or reduced hazards to the equipment, human staff and environment.
2. Description of Related Art
Planarization is a necessary step in the fabrication of multilayer integrated circuits (“ICs”), providing a flat, smooth surface that can be patterned and etched with the accuracy required of modern IC components. The conventional planarization technique is CMP (Chemical Mechanical Planarization or Polishing) known in the art and described in text books (for example, “Chemical Mechanical Planarization of Microelectronic Materials,” by Joseph M. Steigerwald, Shyam P. Murarka and Ronald J. Gutman, 1997). CMP typically makes use of a polishing pad brought into mechanical contact with the wafer to be planarized with an abrasive/reactive slurry interposed between polishing pad and wafer. Typical CMP slurries contain constituents that react chemically with the substrate to be planarized as well as constituents causing planarization by mechanical abrasion. Relative motion of the polishing pad with respect to the wafer leads to polishing of the wafer through mechanical abrasion and chemical etching.
The abrasive slurry polishing materials used in CMP typically comprise an abrasive such as silica, alumina, or ceria and chemically reactive ingredients. Typically, in a practical production environment, these slurry materials cannot be entirely confined to the polishing pad and often splash onto various portions of the CMP apparatus and dry in place leading to increasing deposits of CMP slurry materials at various locations within and on the CMP apparatus itself. Deposits also tend to accumulate on the interior surfaces of delivery tubes and other means used for directing the CMP slurry to the necessary sites. As such deposits build over time, it is a common problem that portions of such slurry deposits break loose in the typical form of abrasive particles that can fall back onto the polishing pads, wafers and/or platens. These unwanted abrasive particles dislodged from slurry deposits on the CMP apparatus are a source of concern to the CMP engineer in that they may cause, inter alia, uncontrollable wafer scratching. Additionally, some slurry polishing materials such as ferric nitrate slurries typically used for CMP of tungsten layers cause unsightly stains on CMP apparatus. Hence, slurry materials and other substances have to be periodically removed from CMP apparatus where they have been deposited. Whether the removal occurs after each pad change, once a week, once a month, or pursuant to any other maintenance schedule does not negate the fact that the apparatus eventually has to be thoroughly cleaned so that slurry and other unwanted extraneous materials are removed therefrom.
Polishing slurries containing ferric nitrate Fe(NO3)3 tend to form ferric hydroxide (Fe(OH)3) and ferric oxide (Fe2O3) residues that tend to precipitate within the conduits of slurry distribution systems that carry slurry to individual CMP apparatus and to specific locations within CMP equipment. These flocculent precipitates can break free, traverse through the slurry distribution system and come into contact with the wafers. Undesirable wafer scratching may result. Hence, there is a need for a cleaning composition effective for removing deposits from the interior regions of the slurry-carrying distribution systems associated with CMP apparatus.
Current practice makes use of surfactant solutions for cleaning CMP apparatus. These solutions are typically not chemically designed to break off or to dissolve typical CMP residues. Thus, the residues must be scraped off with, for example, Teflon scrappers. This scraping technique is both laborious and time-consuming, and results in the generation of loose particles that may fall back onto the apparatus and eventually back onto polishing pads, wafers and/or platen. Uncontrollable wafer scratching and reduced product yields are the typical results. Examples of CMP cleaning systems including wiping operations include those commercially available from The Texwipe Company LLC of Upper Saddle River, N.J. for use in cleaning tungsten CMP slurries (TX8606 SCS) and oxide CMP slurries (TX8065 SCS).
Additionally, current methods for cleaning CMP apparatus make use of hydrogen fluoride (HF) solutions and/or potassium hydroxide (KOH) solutions. Typically, concentrated HF (49%), diluted HF (1% to 10%), or a dilute solution of KOH (e.g., a 5% to 10% solution) is employed. The HF solution or KOH solution is typically sprayed onto the various surfaces of the CMP apparatus that require cleaning and subsequently rinsed with de-ionized (“DI”) water. The use of hydrogen fluoride (HF) for cleaning CMP apparatus has serious disadvantages as a result of this substance being extremely deleterious to the human bone structure if absorbed through the skin and as a result of it also being deleterious if inhaled. HF also requires special disposal methods. Thus, stringent precautions must be sustained when working with and disposing of HF. Additionally, HF fails to successfully remove some chemical stains such as ferric nitrate. Furthermore, HF can also damage the platen plates of CMP apparatus.
A multiplicity of disadvantages are also associated with the use of potassium hydroxide (KOH) for cleaning CMP apparatus. For one thing, KOH fails to successfully remove some chemical stains such as ferric nitrate. Additionally, KOH often leaves a residual composition of potassium, a mobile ion, on the apparatus and polishing pad that may contaminate semiconductor wafers thereby resulting in detrimental effects on the electrical performance of the device and a reduction in yield. Furthermore, KOH can also damage the platen plates and some of the material used to form shields of the CMP apparatus (typically Lexan). Although less dangerous to humans than HF, KOH is nevertheless caustic and requires care in handling and disposal.
Thus, there is a need for a cleaning composition for removing unwanted deposits from CMP apparatus that ameliorates or overcomes one or more of the shortcomings of the prior art. The cleaning composition of the present invention is substantially free of HF, KOH. Furthermore, the present invention does not make use of HCl, in contrast to the work of Thurman-Gonzalez et. al. (WO 99/23688).